Hi Matthew,
I will need to see the new certificate and know how it was generated to determine why it's acting like this, however if your concern now is to get the GUI back online you can enter the following command to change the management certificate to the default one:
ACS81/acsadmin# acs-config
Escape character is CNTL/D.
Username: acsadmin ----------> GUI username
Password: -----------> GUI password
ACS81/acsadmin(config-acs)# reset-management-interface-certificate